style overrides: subtitle styling override
style overrides: do not use user font scale coefficient for positioned events
style overrides: scale borders with user font coefficient
style overrides: make font size style override independent of script resolution

This comes into effect only when enabling selective style overrides.
This isn't used when using forced styles, or when playing SRT subs with
mplayer.

To solve: don't require the user to enable selective style overrides when
rendering SRT subs with mplayer to get the same font size as with
ASS subs + selective style overriding. (The point of selective style
overrides is to make ASS subtitles look like SRT ones in the first place.)
style overrides: use user style's margin values when doing selective style overrides
style overrides: selective style override doesn't apply to events with margins
style overrides: don't apply line position user setting to events with no style overriding

A previous commit added the possibility that a normal (unpositioned) event
is considered as not suitable for selective style overriding. This makes it
necessary to add a special case to the user line position code to leave
these events unaffected.
Revert "style overrides: use user style's margin values when doing selective style overrides"

This reverts commit 0a07dfe28c16d9ae975d0a3b2898bfbbfcc653a9.